1. Field of the invention
This invention relates generally to the field of seismic data interpretation. In particular the invention relates to a machine process for selection of three-dimensional (3D) seismic data to provide petroleum exploration professionals more detailed understanding of subsurface geology and geometry. Still more particularly, this invention is an automated method of "picking" or "tracking" individual seismic events or horizons through a three-dimensional volume of data with extreme accuracy and extreme speed.
2. Description of the prior art
FIGS. 1 through 4 of the Drawings illustrate features and methods associated with the prior art picking methods; FIGS. 5 through 8 illustrate features and methods of this invention. Only Figures associated with prior art methods are introduced here.
FIG. 1 illustrates a portion of a hypothetical 3D seismic data volume in order to explain the three-dimensional relationships discussed in the text and accompanying drawings in this specification;
FIG. 2 is an isometric view of a portion of five seismic traces which illustrates the relationship between a "seed point" and its four adjacent traces;
FIG. 3 illustrates a prior art automatic tracking method; and
FIG. 4 illustrates a prior art "iterative" autotracking method.
FIG. 1 is an isometric view of a portion of a hypothetical three-dimensional (3D) seismic data volume. The small circles at the top of the volume represent the surface location of individual traces. The vertical lines represent seismic traces which are measured in 2 way travel time along the z-axis of the volume. Such travel is related to the distance or depth into the earth at which a wavelet is generated. Each individual trace is an amplitude versus time representation of acoustic reflections from strata in the earth. The graphical view of FIG. 1 is merely a visual representation of the manner in which each seismic trace is actually represented. Each trace is stored as a sequence of digital numbers representing the amplitude of the trace about a zero value. Each number uses many "bits" (a bit is a binary digit having values .phi. or 1) to adequately represent the number which corresponds to the amplitude. Eight, sixteen or thirty-two bits are often used. Of course, such bit representations are repeated for each time point, for example at 2 or 4 milliseconds (m sec) intervals for six total seconds.
A horizontal section or time slice is a horizontal slice or plane through the 3D volume of data. It illustrates different strata at a common time. On the other hand, a horizon map, or simply a "horizon" is obtained by plotting an attribute of a particular wavelet (usually time of the wavelet, but sometimes maximum or minimum amplitudes) on x-y axes. It is similar to a surface topographic map, but of course such a plot is of subsurface strata. The horizon attribute may be illustrated by colors or by line contours etc.
In less than ten years, computer aided exploration revolutionized seismic exploration and field development. Until recently, however, one aspect of seismic interpretation- picking subsurface horizons, or simply, "picking", remained essentially unchanged from paper and pencil methods to automatic computer picking methods.
Traditionally, picking was done manually by drawing with colored pencils on paper, one seismic section or line at a time--an incredibly tedious process. In the early 1980's, interactive CAEX (an acronym for Computer Aided Exploration) workstations gave seismic explorationists the ability to pick 3D data more quickly and effectively. While interpreting seismic lines (that is, a two-dimensional vertical slice or a "vertical seismic section") was still accomplished by viewing and picking one line at a time, it could then be done by using a mouse in combination with a display screen and clicking the cursor on a few selected points along a horizon and letting the machine pick all the rest of the points on that line. This was the first type of automated picking, and represented an incremental increase in both productivity and accuracy over manual picking.
In one prior art automatic system for tracking a bedding plane (or strata or simply "horizon") in a substantially horizontal direction through a 3D volume of data, a user selected or "input" at least one "seed point", which then "expanded" in all four directions within the 3D data volume as illustrated in FIG. 2 until it reached the boundaries of a user specified zone. Users had the option of tracking seismic data in one of two modes.
A "seed point" is specified by its x and y location and its time or depth (i.e., the. z-axis of FIG. 1). It is also specified by a characteristic or attribute of the reflection at that point. Such characteristic is usually the maximum amplitude of the reflection at that location in the volume of the data. Other attributes or characteristics, such as minimum amplitude, phase, frequency, etc., of the reflection at the x, y, z point may be used. As illustrated in FIG. 3, non-iterative tracking searched the seismic traces adjacent seed points for similar amplitude values, picked the best one, then proceeded to the next available trace without double-checking the accuracy of the pick.
An iterative picking mode verified an adjacent trace as a pick by cross-referencing the previous trace. Once verified, the adjacent trace was treated as a seed point and the picking of adjacent traces from it proceeded. FIG. 4 illustrates such prior art iterative picking. Verification means that if the amplitude of the picked trace is within the limits of tolerance set by the user, the pick is accepted. Users could specify (on a scale of 1-10) the degree of amplitude similarity they would allow. If a pick did not pass this acceptance test, it was designated "dead" until at least one directly adjacent trace matched sufficiently to accept it.
More specifically, once a seed point is selected on a trace, the trace is scanned up and down the z or time axis to find the local extreme amplitudes or simply "extrema". A local extremum of a variable x.sub.i where i is a digitizing index, is defined as EQU X.sub.i-1 &lt;X.sub.i .gtoreq.X.sub.i+1
or EQU X.sub.i-1 &gt;X.sub.i .ltoreq.X.sub.i+1.
Such scanning is bounded by zero crossings of the amplitude of the trace in the case of a peak or a trough. Such extremum will typically vary with time a small amount. For example, if T.sub.O represents the seed point, T.sub.1 would typically represent the time of the extremum. Next, the time T.sub.O is started on the target trace. On it, the time is varied up and down between zero crossings of its trace amplitude until the nearest extremum T.sub.2 is found. Finally, the time T.sub.2 is used on the trace on which the seed point exists and on such "seed" trace scanning up and down the "z" axis is again performed for the nearest extremum T.sub.3. If T.sub.3 equals T.sub.1, then iterative tracking has been achieved and tracking continues.
The acceptance test tolerance of the prior art iterative tracking defined a function, ##EQU1## A.sub.t =Amplitude from the target trace at T.sub.2, and A.sub.s =Amplitude from the seed point at T.sub.1.
The value of S is bounded by values of 0 and 1. The more similar the two amplitudes, the closer the S function is to zero. The more dissimilar the two amplitudes, the closer the S function is to 1. Next, a score function is evaluated: EQU SCORE=(S*9.0)1.
The score is compared with a control value from 1 to 10 selected by the interpreter or user of the data. Scores greater than the control value prevent a target trace from being picked.
The prior art techniques described above must process extremely large amounts of data in order to produce or pick a horizon map. Not only must the picking procedures be performed, but their performance requires operation on digital data comprising many bits representative of analog seismic signals. As a result, even with very powerful computers in workstations, a geologist or geophysicists who uses a workstation having a horizon picking program for picking 3D volumes must wait until the program picks through the data and performs the above described picking procedures. Such wait may inhibit creativity where a user desires to view multiple horizons in a short time.